Oh, you must also have a decent source of radiation handy, U ore, radium dial, etc.

The supply need not be able to supply more than 1ma at most.
The audio amp can be any small amp or even a hi-fi amp be sure to use the high level input that needs to be about 1v P-P and not a low signal preamp input or you will overdrive the amp.
The GM tube can be of any type as the supply will work any GM tube over about a 500-1200 volt range normally found in these tubes.
The resistor needs to be physically large, (long) to stand off and evenly distribute voltage. This why a small series string is recommended.

The image below is obvious as it is diagramatic in nature and not schematic. This was produced specifically for one type of tube, but is completely generic in nature for all tubes.

Be sure to turn up the volume on the amp so that the onset of GM action can be heard.
Once the clicking due to a nearby source of radiation is heard, you should be able to safely advance the supply another 50 volts to put you well onto the geiger plateau region. Never raise the voltage more than 100 volts over the onset of clicking.

If you span the full voltage of the supply and get no clicking, then the GM tube is bad.

Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

I have measured plateaus as low as 300 volts and as high as 1200 volts. A normal new 2" pancake has a plateau that is almost always in the 800-950 volt range. Un-quenched tubes using air are often found with very high plateaus. Much depends on the gas fill and the pressure of that fill, tube geometry and whether the tube is hydocarbon quenched, halogen quenched or un-quenched.

The amateur scrounger buying surplus at hamfests, off ebay and obtaining older foreign of soviet bloc tubes may encounter almost anything in the way of GM plateau.

Old worn out tubes can have short plateau lengths, yet, still be servicable to the fine tuned hand.

Richard Hull

Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.